Arsenal have actually being doing pretty good even in times Jesus wasn't able to score any goal. They went on winning by having productive matches in the English Premier League for example. Jesus hasn't been scoring a single goal in all tournaments since October 1. This must have affected his self-confidence in a bad way as well. Now this long running injury is even worse for him.

Arsenal can still go for a temporary striker in January of course. They will have only Nketiah as natural striker in the squad while Jesus is injured. They can secure themselves by doing this. Because if Nketiah also gets injured they would be in trouble.